1. Technical Field
The present disclosure relates to camera modules, methods of assembly of camera modules and imaging devices, in particular relating to camera modules with the image sensor in a cavity in a substrate, such as a printed circuit board.
2. Description of the Related Art
Camera modules, such as those used in mobile phone handsets, have a lens module mounted above an image sensor. The focal plane is at the surface of the image sensor and is known as the image plane.
One of the key criteria of optical design is the Effective Focal Length (EFL) of a lens system. In a camera modules, the EFL is the distance from principal plane of the outer lens to the image plane. Related to EFL is the Total Track Length (TTL), which is the distance from the center of the outer lens to the image plane.
Broadly speaking, the greater the EFL or TTL the better the lens design that can be made. Therefore, every effort is made to place the image plane of the sensor at the lowest possible position in a module (oriented with the lens at the top) to maximize the space available for the lens such that the lens characteristics can be optimized with the longest EFL (or TTL).
One approach that has been used in camera module construction is shown in FIG. 1 where the image sensor 10 is placed on top of a printed circuit board (PCB) 12 (herein referred to as a substrate) and is surmounted by a lens module 14 screwed into a mount 16 with an infra-red filter 18 (typically a specially coated glass tile). Bond wires 20 are also shown electrically connecting the image sensor to the substrate.
FIG. 2 shows the assembled camera module of FIG. 1. The vertical scale is exaggerated, particularly at the lower end of the mount 16 and for the infra-red filter 18 and image sensor 10. The image sensor 10 is placed on and attached to the substrate 12. The image sensor has an imaging area (e.g., a pixel array) 22, illustrated here as having some volume extending from the surface into the image sensor. The mount 16 is attached to the substrate 12 and the infra-red filter is attached to the mount 16.
In this construction, the EFL (or TTL) is limited by the distance between the top of the camera module and the top of the image plane (upper surface of the sensor). If the height of the image plane could be lowered, then there would be more room for the lens, the EFL (or TTL) of the lens could be increased resulting in improved optical performance.
Typically camera modules using a construction as shown in FIGS. 1 an 2 rely on minimizing both the sensor thickness and the substrate thickness to reduce image plane height. This may for example permit an image plane height of as low as 480 μm.